TORNADOES

Tornadoes occur more frequently and are
more intense in the Great Plains than in any
other region on earth. Plains geography, with
fringing mountain ranges running predominantly
north to south, allows cold dry air from
the north to collide with warm moist tropical
air from the south. The jet stream, which seasonally
migrates south across the Great Plains,
is another essential ingredient in the creation
of tornadoes. The highest frequency of tornadoes
is in central Oklahoma, but incidence
is also high in a zone from northeast Texas
through Kansas to eastern Nebraska and Iowa.
This region is commonly known as "tornado
alley." The tornado season for North America
peaks in May; however, there is considerable
geographic variation in the timing of peak occurrence.
For example, tornadoes in Texas
and Oklahoma peak in April, in Nebraska in
June, and in the Southern Prairie Provinces in
late July and early August.

Tornadoes develop on the backside of particularly
strong thunderstorms and hang down
from the cloud as if the thunderstorm had a
tail. These tornado-producing thunderstorms
can occur singly or in a line known as a squall
line, which often produces multiple tornadoes.
Great Plains tornadoes commonly occur in
late afternoon, when surface temperatures are
the highest, providing the daily maximum uplift
to the moist air near the ground. Most
tornadoes move from the southwest toward
the northeast; however, they may travel in any
direction, and in some rare instances they remain
stationary or reverse direction.

Tornado intensity is described by the fscale.
F0 and F1 tornadoes are weak tornadoes,
with top wind speeds reaching 112 miles per
hour, a typical width of 50 to 100 feet, and a
typical life span of only a few minutes. They
account for 68 percent of all Great Plains tornadoes.
F2 and F3 tornadoes are strong tornadoes,
with winds as high as 206 miles per
hour, a width of 100 to 400 feet, and a life span
of more than twenty minutes. They account
for 30 percent of all Great Plains tornadoes. F4
and F5 violent tornadoes are the strongest,
with wind speeds that can exceed 300 miles per
hour, a funnel as wide as a mile, and a duration
of several hours. Violent tornadoes represent
only 2 percent of all tornadoes in the
Great Plains, but they account for more than
90 percent of tornado-caused deaths. For the
period 1950–97, the annual average number of
F2 to F5 tornadoes was twenty-nine for Texas,
seventeen for Oklahoma, eleven for Kansas,
seven for Nebraska, five for South Dakota,
three for North Dakota, and about one per
year for each of the Prairie Provinces.

Average annual tornado incidence per 10,000 square miles from 1970 to 1997

Tornado data have been collected only since
the early 1950s. Since that time the data indicate
that there has been an increasing trend in
tornadoes in the Great Plains. However, this
increase has been observed only in the weak
F0 and F1 tornadoes; there has been no increase
in the number of days with tornadoes.
Scientists have concluded that almost all of the
apparent increase is due to enhanced public
awareness and better reporting methods during
the last few decades.